Resistor composition



March 16, 1965 o. A. SHORT 3,173,885

RESISTOR COMPOSITION Filed July 11. 1962 INVENTOR OLIVER A. SHORT ATTORNEY United States Patent 3,173,885 RESISTOR COMPQSITION Oliver A. Short, Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed July 11, 1962, Ser. No. 209,176 4 Claims. (Cl. 252-503) This invention relates to new and improved resistor compositions and low temperature flexible heaters produced therewith.

Heretofore many attempts have been made to produce resistor compositions that can be applied to light flexible materials such as paper, cellophane, polyethylene, polypropylene and other resinous polymeric films to make low temperature heaters. Such flexible low temperature heaters are demanded in making electric blankets, radiant room heater panels, ice cube release devices, defrosters, radiant room heater wall paper and the like.

Silver has been prominently mentioned as a suitable conductive material and numerous resinous polymeric materials have been suggested as carriers for the conductive material. Although previously constructed low temperature flexible heaters have been operative they have not been fully satisfactory from a standpoint of adequate resistance and long continued use under repeated flexing conditions.

In the first place, such resistances must be applicable to common heat sensitive flexible materials such as paper, polyethylene or other synthetic base films at a low temperature and under conditions that will not be harmful to the strength of such materials.

Secondly, it must be strongly bonded to the flexible base and it must remain highly flexible without the slightest rupture in the resistance layer.

Thirdly, it must have a properly adjusted resistance value when dried.

It is an object of this invention to provide an improved resistor composition applicable at low temperature to flexible sheet material to form a low temperature flexible heating element.

It is another object to provide a resistor composition applicable at low temperature to flexible sheet material to form a low temperature flexible heating element having a properly adjusted resistance value.

It is still another object to provide an improved resistor composition which when applied to a flexible sheet material will be strongly bonded thereto and will remain strong and flexible under continuous flexing conditions.

It is another object to produce flexible low temperature heaters comprising flexible sheet material and a resistance element strongly bonded to the sheet material to permit repeated flexing Without rupture.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished by preparing a resistmce composition by mixing together finely divided silver, graphite and a binder composed of between 30% and 70% of an alkyl methacrylate polymer and between 70% and 30% of a copolymer of 60% to 75% ethylene and 40% to 25% vinyl acetate, in the proportions shown in area ABCD in the accompmying ternary diagram. Such resistor compositions can be coated on flexible sheet material composed of paper, polyethylene, cellophane, polypropylene or other tough flexible synthetic resinous film to produce long lasting highly satisfactory low temperature heaters.

Since the heating elements in refrigerators, freezers, electric blankets and the like require wattages of from about 2 to 120 watts and since the normal home voltage is 110 AC, the resistance path should have a resistance of between about 0.1 and ohms per square and a our- "Ice rent carrying capacity of between about 0.5 and 12 amperes per inch of width.

Throughout the specification, reference to ohms per square refers to conductive coatings having a thickness of approximately 1 mil.

A resistance pattern produced with silver in a resinous binder has a resistance around 0.04 ohm per square whereas a similar resistance pattern with carbon as the conductive material has a resistance of around 100 to 200 ohms per square. In order to obtain a resistance in the desired range it is necessary to combine 25% to silver with 5% to 70% carbon and 5% to 35% of the aforesaid binder.

The binder comprises a combination of an alkyl methacrylate polymer and a copolymer of ethylene and vinyl acetate. The alkyl methacrylate polymer may be meth yl, ethyl, isobutyl or normal butyl methacrylate polymer. The ethylenevinyl acetate coplymer is one containing between 25% and 40% vinyl acetate units in the polymer. The proportions of the methacrylate polymer and ethylene-vinyl acetate copolymer in the binder may vary be tween 30% methacrylate polymer and 70% of the copolymer to 30% of the copolymer and 70% of the methacrylate polymer.

A binder composed of only one or the other of the above polymers is quite unsatisfactory for the production of the flexible printed resistors of this invention. The use of ethylene-vinyl acetate copolymer alone is unsatisfactory since it is very difiicut to use especially with screen printing and especially with the presence of finely divided carbon. The use of a methacrylate polymer alone is unsatisfactory because the resulting film is not sufficiently flexible and will rupture after a period of use.

In the drawing accompanying this description, FIG. 1 shows a ternary diagram in which curves F, G, H, I, J, K, L, M and N show the variations of silver, graphite and binder to produce a dry film having specific resistances in ohms per square, as indicated. The area ABCD designates the area on the ternary diagram containing the relative proportions of silver, graphite and binder in the resulting dry film that will produce the uniquely operative compositions of this invention and have resistances substantially within the range of 0.1 to 10 ohms per square and a current carrying capacity of between about 0.5 and 12 amperes per inch of Width.

Throughout the specification, specific reference is repeatedly made to graphite as the source of carbon. It is to be understood that other forms of carbon such as lampblack and charcoal can be used equally well.

The carbon and silver are in a finely divided form preferably of a size to pass through a 200 mesh or finer screen. The carbon, silver and binder are usually incorporated in a vehicle and are applied to the flexible base film by screen stencilling.

As vehicles for the finely divided silver, graphite and binder slow drying materials having a good solvent action for the binder may be used, for example, mono ethylene glycol butyl ether acetate, mono ethylene glycol ethyl or phenyl ether acetates, diethylene glycol mono ethyl ether, diethylene glycol mono ethyl (or methyl, propyl, butyl) ether acetates, pine oil, alpha or beta terpineol, terpene others, turpentine, or petroleum solvents that will dissolve the methacrylates. In order to obtain the requisite viscosity of suitable screen stencilling, the vehicle should be present in about 15% to 35% by weight.

The following tabulated dried compositions of silver, carbon and binder composed of 50% ethylene-vinyl acetate copolymer of 28% vinyl acetate content and 50% of methyl methacrylate polymer on a flexible film will constitute resistances of the values indicated. The particu- 3 lat composition of binder used will have very little efiect on the resistance values.

Many other examples may be chosen from the area of the chart that is traversed by the constant ohms/ square contours. The maximum amps/inch of each possible composition is not known, but they all lie above 1.0 if

the binder is kept below 20% and are probably above 0.5 if the binder is below 35%. The upper limit for amps/inch is not fixed since a substantial safety factor should be allowed.

After printing and drying, the pattern must baconnected electrically to the outside circuit. Several efifective methods are available for this.

(1) Wires may be cemented to the ends of the pattern with air drying or thermosetting silvers.

(2) Patches of solderable silver may be applied to the ends of the pattern and the wire leads soldered to these patches. 7

(3) Wires may be crimped through holes in the 'film in such a manner that they contact the pattern.

(4) Wires may be flattened or soldered to brass lugs which are placed on the pattern and held in place during the succeeding laminating process.

Although the device will act as a heater after the leads are attached, it is preferable to laminate or coat the circuit to obtain good electrical insulation. Again', several methods are available to accomplish this result.

(1) The entire heater may be coated with a mixture Throughout the specification and claims, any reference to parts, proportions and percentages-re'fers-to parts, proportions and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

What I claim is:

1. A compositionsuitable for application to a flexible film to produce an electrical resistor consisting essentially of finely divided silver, carbon and binder in the proportions shown by the area ABCD of the accompanying ternary diagram, said binder consisting essentially of between 30% and 70% of an alkyl methacrylate polymer taken from the group consisting of methyl, ethyl, isobutyl and normal butyl methacrylate polymers and between 70% and 30% of a copolymer of 60% to 75% ethylene and 40% to 25% vinylacetate.

2. A composition suitable for application to a flexible film to produce an electrical resistor consisting essentially of finely divided silver, carbon and binder consisting essentially of between 30% and 70% of an alkyl methacrylate polymer taken from the group consisting of methyl, ethyl, isobutyl and normal butyl methacrylate polymers and between 70% and 30% of a copolymer of 60% to 75% ethylene and 40% to 25% vinyl acetate, said materials being present in the proportions of 25% to 90% silver, to 70% carbon, and 5% to 35% binder.

3. The composition defiined in claim 2 containing between and 35% of an air-drying vehicle having a solvent action on said binder. I

4. An-electrical resistor consisting essentially of a flexible film and a printer pattern consisting essentially of finely divided silver and carbon in a binder matrix in the proportions of to 90% silver, 5% to 70% carbon and 5% to binder, said binderconsisting 'essentially of between 30% and of an alkyl methacrylate polymer taken from the group consisting of methyl, ethyl, isobutyl and normal butyl methacrylate polymers and between 70% and 30% of a copolymer of 60% to and 30% to 25% vinyl acetate.

. References tilted by thefExaminer UNITED STATES PATENTS 2,545,229 2/51 Chapman. 2,825,702 3/58 Silversher 252 5o5 3,056,750 '10/62 Pass 252 512 JULIUS GREENWALD, Primary Examiner. 

1. A COMPOSITION SUITABLE FOR APPLICATION TO A FLEXIBLE FILM TO PRODUCE AN ELECTRICAL RESISTOR CONSISTING ESSENTIALLY OF FINELY DIVIDED SILVER, CARBON AND BINDER IN THE PROPORTIONS SHOWN BY THE AREA ABCD OF THE ACCOMPANYING TERNARY DIAGRAM, SAID BINDER CONSISTING ESSENTIALLY OF BETWEEN 30% AND 70% OF AN ALKYL METHACRYLATE POLYMER TAKEN FROM THE GROUP CONSISTING OF METHYL, ETHYL, ISOBUTYL AND NORMAL BUTYL METHACRYLATE POLYMERS AND BETWEEN 